I think "not reinventing the wheel" goes both ways, if you use smartBASIC you'll need new code for your sensors but all the communication inc LoRa BLE and peripherals, low power modes and certification is done for you.
If you go the other way maybe - e.g. if not timing critical - you can reuse your sensor code but you'll have to setup your peripherals, handle your LoRa stack (I'm not sure even the mbed version is a direct drop in), do power management, BLE and of course lose the certification.
It all depends on what your sensor code does and also what you need from the module. Maybe you don't need low power modes and/or BLE and that certainly makes it a bit simpler.
I've been tempted to "roll my own" stack mainly because smartBASIC doesn't support everything the nRF51 can (please Laird support low power comparators!) but decided to keep my sanity instead and go with smartBASIC. Once you get past the "but it's BASIC!" shock I find it much faster to develop at least for relatively simple nodes.